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Many states have recently increased the stringency of their residential energy codes, forcing builders to rethink long-established construction practices. In some areas, contractors who have always built houses with 2x4 walls and uninsulated basements are waking up to new regulations requiring basement wall insulation and much higher R-values for above-grade walls. Elsewhere, building officials have begun checking the U-factors on window labels for the first time.

Energy codes vary widely from state to state. While many states require residential builders to comply with the Inter-national Energy Conservation Code (IECC), the successor to the old Model Energy Code (MEC), other states — including Alabama, Arizona, Colorado, Illinois, Mississippi, Missouri, and South Dakota — have no statewide residential energy code.

Even when a state decides to adopt the IECC, however, plenty of opportunities for confusion remain. At least five different versions of the IECC are currently being enforced in the United States. The most recent version, the 2006 IECC (adopted by Iowa, Louisiana, Pennsylvania, and Utah), is radically different from earlier versions of the IECC enforced in several other states.

Moreover, a number of states have adopted the IECC with state-specific modifications. For example, New Jersey's code permits builders to omit basement wall insulation in any home equipped with a 90 percent AFUE (or better) furnace; New York, on the other hand, specifically prohibits any design with a trade-off that eliminates basement wall insulation.

Several model residential energy codes are currently in print, including the 1992 and 1995 MECs, and the 2000, 2003, and 2006 IECCs; code books are available at prices from $11 to $31 from the International Code Council (

Forty-four states now enforce an energy code based on either the MEC or a pre-2004 version of the IECC (see "Residential Energy Codes by State"). These codes allow builders to choose from three compliance options: a prescriptive path, a component trade-off path, and a systems analysis path.


The Prescriptive Path

Dubbed the "cookbook" path in Minnesota, the prescriptive path is the simplest — though not necessarily most cost-effective — way for builders to meet energy-code requirements. Prescriptive-path requirements usually include minimum R-values for insulation, with different R-values specified for walls, ceilings, floors, basement walls, and slab edges. Some prescriptive codes also specify a maximum U-factor or a maximum solar heat-gain coefficient (SHGC) for windows.

Prescriptive-code requirements are usually shown in a table (for example, Table 602.1 in the 2000 IECC) that specifies minimum R-values, maximum U-factors, and maximum SHGC values; these prescribed values typically vary by climate zone or by the number of heating degree days at the building site.


Prescriptive tables, like this one in the 2000 IECC, provide a cookbook approach to energy design but may not result in the least expensive building.

Windows from major manufacturers are labeled with U-factor and SHGC values calculated according to procedures established by the National Fenestration Rating Council (NFRC). If a window lacks an NFRC label, builders must use code-specified "default values" when demonstrating code compliance; for example, a vinyl window with double glazing is assigned a default U-factor of 0.55.

In some regions of the country, the best available default window U-factors or SHGC values aren't low enough to satisfy the prescriptive code, so NFRC-labeled windows are the only option open to builders following the prescriptive path.

In pre-2004 versions of the IECC, builders following the prescriptive path need to calculate the home's window-to-wall ratio (WWR). Homes with a WWR of 15 percent or less should follow the prescriptive tables in Chapter 6 of the code, and homes with a WWR of more than 15 percent need to follow the prescriptive tables in Chapter 5. Builders must include rim joist areas in wall-area calculations; window areas are based on rough-opening areas.

The idea of the WWR originated in the original 1992 MEC (see "Making Sense of the Model Energy Code," 11/99). In pre-2004 versions of the IECC, all three compliance paths require builders to calculate the WWR. (In Washington state, the residential energy code requires builders to calculate a different ratio, the window-to-floor-area ratio.)

The Component Trade-Off Path

Because the prescriptive path is inflexible, its use often results in a house that costs more to build than a house that follows the component trade-off path. Builders who choose the component trade-off path are able to adjust several variables — such as insulation thickness, window area, or furnace efficiency — in search of the most cost-effective way to comply with energy-code requirements. In pre-2004 versions of the IECC, the component trade-off path is found in Chapter 5.

In some states, the component trade-off path is called the component performance path or — somewhat confusingly — the performance calculation path. However, this path does not involve a full-fledged calculation of a home's energy performance; rather, it involves a simplified performance calculation based on a limited number of trade-offs.

For example, many state energy codes allow a house equipped with a high-efficiency furnace to skimp on wall or ceiling insulation. The rationale behind such a trade-off is simple: Although the resulting house has different specifications than a house following the prescriptive path, the two houses cost about the same to heat.

"The energy codes don't really require minimum levels of insulation," says Joe Nagan, technical director for Wisconsin Energy Star Homes. "For example, in their prescriptive insulation tables, the codes generally assume that you have a 78 percent AFUE furnace. But as long as your trade-off gives you a heat loss that is less than the maximum allowable heat loss, you pass. If you don't pass, you can either beef up the walls or you can go to a more efficient furnace."

In states with an energy code based on the 2004 IECC or earlier model codes, adjustments in window area can be used as a trade-off. For instance, thicker attic insulation or better-performing windows can be used as a trade-off for a high window-to-wall ratio; conversely, a low WWR may allow builders to skimp on insulation.

The easiest way to follow the component trade-off path is to use computer software — for example, a free program called REScheck — to fine-tune a home's specifications. Although first-time users of REScheck may be intimidated by the software, most builders soon navigate the program with ease.

Using REScheck

REScheck is a software tool used to demonstrate that a house design complies with residential energy codes. The program was developed by the U.S. Department of Energy, and can be downloaded at no charge from

Not all states allow the use of REScheck for demonstrating energy-code compliance, so it's important to check local code requirements before deciding to use REScheck. Florida builders usually show code compliance with EnergyGauge software, while California energy consultants use one of several California-specific software tools to meet the state's Title 24 requirements. Among the states that do allow the use of REScheck are Arkansas, Georgia, Massachusetts, Minnesota, New Hampshire, New Jersey, New York, Vermont, and Wisconsin.

Once you have REScheck loaded on your computer, you're ready to see if your house design meets code. At the "Code" tab, choose the code you will be complying with — for example, MEC or a particular version of the IECC. If you live in a state with a state-specific code, it's important to indicate (at the "Code" tab) the state where the house is being built.

The program has five main tabs to click: "Project," "Envelope," "Mechanical," "Loads," and "Energy Star." Under each tab are boxes where the user enters information about the house in question. Probably the most time-consuming step is calculating the area of the home's components, including floor areas, wall areas, ceiling areas, and window areas.



In this example, a simple 30-foot-by-40-foot ranch house has R-19 wall insulation, R-38 ceiling insulation, R-5 basement wall insulation, and windows with a U-factor of 0.34. If the house is equipped with a 78 percent AFUE furnace, it does not pass Wisconsin code. On the REScheck program, the line at the bottom of the screen indicates code compliance or failure; in this case, it indicates "Fails" and "9.2% Worse Than Code."

After entering the required information, including the insulation R-values, it's time to click the "Check compliance" button in the lower left-hand corner. The program then indicates whether your design "Passes" or "Fails," and displays the percentage by which it either exceeds or falls short of your energy code (for example, "Your UA is 2.6% better than code," or "Your UA is 16.2% worse than code"). By changing the home's insulation values or window sizes, an out-of-compliance home can be brought into compliance (see screen shots).



The same ranch house in Wisconsin becomes code-compliant when the basement wall insulation is increased to R-10. The REScheck program indicates "Passes" and "4.6% Better Than Code."



Instead of increasing the thickness of the attic and basement wall insulation, a builder could swap the 78 percent AFUE furnace for a 92 percent AFUE furnace. That upgrade results in a house that is 7.4 percent better than code — even with the original R-5 basement wall insulation.

In most cases, REScheck determines code compliance by calculating the home's UA. (UA is the overall average heat transmission of the area of a building's exterior envelope; that is, the average U-factor of the envelope times the area of the entire envelope.)

If the home includes high-efficiency hvac equipment, REScheck can (in certain states, or for some model codes) perform a limited-scope performance analysis. However, use of the performance path is not always advantageous to a builder. Under the 2006 IECC, the performance path calculation considers glazing area and orientation, so a home that is not advantageously oriented (from a solar perspective) may fail worse when following the performance path than it did using the prescriptive path.

Here, then, are a few facts to remember:

• When entering wall areas into REScheck, use gross wall areas (including band-joist areas), not net wall areas. REScheck automatically subtracts the area of the windows and doors to calculate net wall areas.

• When entering window areas, enter either the rough opening area or the window frame area, not the sash area or the glass area.

• REScheck automatically adjusts R-values as required to account for drywall, air films, and the like, so enter only the R-value shown on the insulation label.

• If you are passing code by means of the UA calculation method, you don't need to enter information on the home's hvac system.

• If you are complying with the 2006 version of the IECC, some compliance paths require specifying the orientation (north, south, east, or west) of the windows and walls.

• REScheck has certain inherent limitations; for example, it is unable to handle a house with more than one heating system.

Anyone with questions about REScheck should explore the resources available online at They include the REScheck Software User's Guide.

While the component trade-off path is popular in northern states, southern builders often stick with the prescriptive path. "Where the prescriptive codes most align with current building practice, builders tend to use the prescriptive codes," says Mike DeWein, technical director for the Building Codes Assistance Project in Washington, D.C. "That tends to be in the warmer climate zones. Where current practice varies from the prescriptive requirements, builders usually want to use the trade-off or the performance method. In a good chunk of the northern half of the country, builders and design professionals are very comfortable with REScheck."

Builders should remember that some trade-off strategies, though code-compliant, may result in an uncomfortable building. For example, many state energy codes allow builders to trade thicker attic insulation for cheaper windows. While the resulting house may satisfy the energy code, high U-factor windows may lead to comfort complaints.